1. Field of the Invention
This invention relates to a radiation image read-out apparatus, and more particularly to a radiation image read-out apparatus for reading out a radiation image stored on a stimulable phosphor sheet by the use of a line sensor.
2. Description of the Related Art
When certain kinds of phosphor are exposed to a radiation, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light or a laser beam, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emissionxe2x80x9d. There has been put into wide use a radiation image recording and reproducing system as a computed radiography in which a stimulable phosphor sheet (a sheet comprising a support sheet and a layer of the stimulable phosphor formed thereon) is exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet, a stimulating light beam such as a laser beam is projected onto the stimulable phosphor sheet, the stimulated emission emitted from the stimulable phosphor sheet upon excitation by the stimulating light is photoelectrically detected, thereby obtaining an image signal, and then the stimulable phosphor sheet is exposed to erasing light after the image signal is obtained from the stimulable phosphor sheet so that the residual energy of the radiation is fully released from the stimulable phosphor sheet.
It has been proposed to allot the radiation absorbing function and the energy storing function of the stimulable phosphor conventionally employed in the radiation image recording and reproducing system between two kinds of phosphor, one being excellent in the radiation absorbing function and the other being quick in response in emitting the stimulated emission upon excitation by the stimulating light. With this arrangement, the radiation absorbing efficiency can be improved and at the same time, the energy storing efficiency can be increased. There also has been proposed a system in which a phosphor excellent in radiation absorbing power is caused to absorb the radiation, another phosphor quick in response in emitting the stimulated emission upon excitation by the stimulating light is caused to absorb light emitted from the phosphor excellent in radiation absorbing power upon excitation by light in ultraviolet to visible region and to store energy, then said another phosphor is excited by light in visible to infrared region to emit stimulated emission, and the stimulated emission is photoelectrically read by a photoelectric read-out means to obtain an image signal. (See, Japanese Patent Application No. 11(1999)-372978.)
The radiation image signal obtained in the systems described above is subjected to image processing such as gradation processing and/or frequency processing and a radiation image of the object is reproduced as a visible image for diagnosis on the basis of the processed radiation image signal on a recording medium such as a photographic film or a display such as a high-resolution CRT. When the stimulable phosphor sheet is exposed to erasing light, the residual energy of the radiation is fully released from the stimulable phosphor sheet and the stimulable phosphor sheet comes to be able to store a radiation image again, whereby the stimulable phosphor sheet can be repeatedly used.
In the radiation image information read-out apparatus employed in the radiation image recording and reproducing apparatus, it has been proposed to use a line light source which projects a line beam onto the stimulable phosphor sheet as a stimulating light source and to use a line sensor having an array of photoelectric convertor elements extending in the main scanning direction (the longitudinal direction of the line beam) as a means for photoelectrically reading out the stimulated emission. The line beam and the line sensor are moved relative to the stimulable phosphor sheet in a sub-scanning direction (the direction substantially perpendicular to the longitudinal direction of the line beam) by a scanning means. By the use of a line beam and a line sensor, the reading time is shortened, the overall size of the apparatus can be reduced and the cost can be reduced. See, for instance, Japanese Unexamined Patent Publication Nos. 60(1985) -111568, 60(1985)-236354, and 1(1989)-101540. In such a radiation image information read-out apparatus, the line sensor is positioned close to the stimulable phosphor sheet and a condenser lens is provided between the line sensor and the stimulable phosphor sheet in order to collect the stimulated emission on the light receiving face of the line sensor.
In the radiation image information read-out apparatus, there has been a problem that a part of the stimulating light projected onto the surface of the stimulable phosphor sheet is reflected by the surface of the stimulable phosphor sheet to impinge upon the line sensor and deteriorates the image in contrast.
In order to overcome this problem, it has been proposed to insert a color glass filter suitable for cutting the stimulating light between the stimulable phosphor sheet and the line sensor. However since the line sensor is positioned close to the stimulable phosphor sheet, and at the same time the condenser lens is provided between the line sensor and the stimulable phosphor sheet as described above, the thickness of the color filter to be inserted between the stimulable phosphor sheet and the line sensor is limited. Accordingly, the stimulating light cannot be satisfactorily cut and the contrast of the image cannot be satisfactorily increased.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radiation image information read-out apparatus in which the stimulating light can be satisfactorily separated from the stimulated emission not to impinge upon the line sensor.
In one aspect of the present invention, there is provided a radiation image read-out apparatus comprising a stimulating light beam projecting means which projects a line stimulating light beam onto a stimulable phosphor sheet, storing thereon radiation image information, to extend in a main scanning direction, a line sensor consisting of a plurality of photoelectric convertor elements which receive stimulated emission emitted from the portion exposed to the line stimulating beam to convert the amount of stimulated emission to an electric signal and are arranged in a row which extends along the line portion of the stimulable phosphor sheet exposed to the line stimulating beam, a condenser lens which is disposed along the line sensor to collect the stimulated emission on the light receiving face of the line sensor and a sub-scanning means which moves one of the line sensor and the stimulable phosphor sheet relatively to each other in a sub-scanning direction intersecting the main scanning direction, wherein the improvement comprises that
said condenser lens has a chromatic aberration of magnification in the sub-scanning direction such that most of the light in the wavelength range of the stimulated emission is collected on the light receiving face of the line sensor and most of the light in the wavelength range of the stimulating light is collected outside the light receiving face of the line sensor.
Generally the wavelength of the stimulating light is longer than that of the stimulated emission. The present invention is characterized in that the stimulating light and the stimulated emission are separated from each other on the basis of the chromatic aberration of magnification generated by the wavelength difference therebetween.
The expression xe2x80x9cmost of the light in the wavelength range of the stimulated emission is collected on the light receiving face of the line sensorxe2x80x9d means that, though a part of the light in the wavelength range of the stimulated emission may go outside the light receiving face of the line sensor, the amount of the light collected on the light receiving face of the line sensor is more than that going outside the same. Similarly, the expression xe2x80x9cmost of the light in the wavelength range of the stimulating light is collected outside the light receiving face of the line sensorxe2x80x9d means that, though a part of the light in the wavelength range of the stimulating light may impinge upon the light receiving face of the line sensor, the amount of the light collected outside the light receiving face of the line sensor is more than that impinging upon the same.
The chromatic aberration of magnification depends upon the size of the light receiving face, the diameter of the condenser lens, the distance between the light receiving face and the condenser lens, and the like, and accordingly, the chromatic aberration of magnification can be controlled to satisfy the aforesaid condition by adjusting the size of the light receiving face, the diameter of the condenser lens, the distance between the light receiving face and the condenser lens, and the like as well as the properties of the condenser lens itself (e.g., the refractive index).
In another aspect of the present invention, there is provided a radiation image read-out apparatus comprising a stimulating light beam projecting means which projects a line stimulating light beam onto a stimulable phosphor sheet, storing thereon radiation image information, to extend in a main scanning direction, a line sensor consisting of a plurality of photoelectric convertor elements which receive stimulated emission emitted from the portion exposed to the line stimulating beam to convert the amount of stimulated emission to an electric signal and are arranged in a row which extends along the line portion of the stimulable phosphor sheet exposed to the line stimulating beam, a condenser lens which is disposed along the line sensor to collect the stimulated emission on the light receiving face of the line sensor and a sub-scanning means which moves one of the line sensor and the stimulable phosphor sheet relatively to each other in a sub-scanning direction intersecting the main scanning direction, wherein the improvement comprises that
the following formula (1) is satisfied,                                           1            ⁢                          (                              m                ⁢                                  xe2x80x83                                ⁢                m                            )                                                          λ              ⁢                              xe2x80x83                            ⁢                              a                ⁡                                  (                                      n                    ⁢                                          xe2x80x83                                        ⁢                    m                                    )                                                      -                          λ              ⁢                              xe2x80x83                            ⁢                              b                ⁡                                  (                                      n                    ⁢                                          xe2x80x83                                        ⁢                    m                                    )                                                                    ≤                              Δ            ⁢                          xe2x80x83                        ⁢                          d              ⁡                              (                                  m                  ⁢                                      xe2x80x83                                    ⁢                  m                                )                                                          Δλ            ⁡                          (                              n                ⁢                                  xe2x80x83                                ⁢                m                            )                                                          (        1        )            
wherein xcexa (nm) represents the center wavelength of the stimulating light, xcexb (nm) represents the center wavelength of the stimulated emission, xcex94xcex (nm) represents the wavelength difference between the longest wavelength and the shortest wavelength of the incident light components on the condenser lens and xcex94d (mm) represents the distance of dispersion on the light receiving face of the incident light components on the condenser lens.
As described above, center wavelength of the stimulating light xcex94a greater than center wavelength of the stimulated emission xcexb, and to satisfy formula (1) means that the difference between the wavelength of the stimulating light and that of the stimulated emission xcexaxe2x88x92xcexb results in the distance of dispersion on the light receiving face between the stimulating light and the stimulated emission not smaller than 1 mm. The ratio of the distance of dispersion on the light receiving face between the longest wavelength and the shortest wavelength of the incident light components on the condenser lens to the wavelength difference between the longest wavelength and the shortest wavelength of the incident light components on the condenser lens xcex94d/xcex94xcex is defined to be the dichroic aberration of magnification, here.
Since the chromatic aberration of magnification depends upon the size of the light receiving face, the diameter of the condenser lens, the distance between the light receiving face and the condenser lens, and the like as described above, the sizes and the distance are set so that the chromatic aberration of the condenser lens satisfies the above formula (1).
It is further preferred that the following formula (2) be satisfied                                           Δ            ⁢                          xe2x80x83                        ⁢                          d              ⁡                              (                                  m                  ⁢                                      xe2x80x83                                    ⁢                  m                                )                                                          Δλ            ⁡                          (                              n                ⁢                                  xe2x80x83                                ⁢                m                            )                                      ≤                              0.4            ⁢                          (                              m                ⁢                                  xe2x80x83                                ⁢                m                            )                                            Δλ            ⁢                          xe2x80x83                        ⁢                          b              /              2                        ⁢                          (                              n                ⁢                                  xe2x80x83                                ⁢                m                            )                                                          (        2        )            
wherein xcex94xcexb (nm) represents the width of the wavelength range of the stimulated emission.
In order to separate the stimulating light and the stimulated emission from each other, the more the chromatic aberration of magnification is, the better it is. On the other hand, when the chromatic aberration of magnification is excessively large, in the case of the stimulated emission whose wavelength range has a certain width, the stimulated emission collecting efficiency in the sub-scanning direction deteriorates since the stimulated emission spreads over a too large area due to the too large chromatic aberration of magnification and at the same time, the stimulated emission emitted from one pixel spreads over a plurality of photoelectric convertor elements arranged in the main scanning direction on the light receiving face of the line sensor, which results in deterioration in sharpness. This problem can be prevented when formula (2), which limits the distance of dispersion for a given width of the wavelength range of the stimulated emission, is satisfied. The above formula (2) means that the distance of dispersion should be not larger than 0.4 mm for a half of the width of the wavelength range of the stimulated emission.
Generally, the width of the wavelength range of the stimulated emission xcex94xcexb is smaller than the difference between the center wavelength of the stimulating light and the center wavelength of the stimulated emission.
It is preferred that the effective width of the photoelectric convertor element as measured in the direction perpendicular to the longitudinal direction of the line sensor be 20 xcexcm to 300 xcexcm. The effective width may be formed by covering with light-shielding material a part of the light receiving face of a photoelectric convertor element having a larger light receiving face.
It is further preferred that a stimulating light cut filter which is permeable to the stimulated emission and impermeable to the stimulating light be disposed between the line sensor and the stimulable phosphor sheet. The expression xe2x80x9cthe stimulating light cut filter is permeable to the stimulated emission and impermeable to the stimulating lightxe2x80x9d does not express that the stimulating light cut filter transmits 100% of the stimulated emission and no stimulating light but only express that the stimulating light cut filter is more permeable to the stimulated emission than to the stimulating light.
It is preferred that the condenser lens be not larger in the distance of dispersion of incident light on the condenser lens in the longitudinal direction of the light receiving face of the line sensor than that in the direction perpendicular to the longitudinal direction of the light receiving face of the line sensor as measured on the light receiving face of the line sensor.
When the condenser lens has a chromatic aberration of magnification in the sub-scanning direction such that most of the light in the wavelength range of the stimulated emission is collected on the light receiving face of the line sensor and most of the light in the wavelength range of the stimulating light is collected outside the light receiving face of the line sensor, the stimulating light is suppressed from impinging upon the photoelectric convertor elements in the sub-scanning direction, whereby the image can be enhanced in contrast.
To satisfy formula (1) means that the difference between the wavelength of the stimulating light and that of the stimulated emission xcexaxe2x88x92xcexb results in the distance of dispersion on the light receiving face between the stimulating light and the stimulated emission not smaller than 1 mm. When the effective width of the light receiving face is sufficiently smaller than the distance of dispersion on the light receiving face between the stimulating light and the stimulated emission, most of the stimulating light can be collected outside the light receiving face of the line sensor in the sub-scanning direction by causing most of the light in the wavelength range of the stimulated emission to be collected on the light receiving face of the line sensor, whereby the stimulating light is suppressed from impinging upon the photoelectric convertor elements in the sub-scanning direction, and the image can be enhanced in contrast.
When the aforesaid formula (2) is satisfied, the stimulated emission is suppressed from going outside the light receiving face due to the chromatic aberration of magnification of the condenser lens and from impinging upon a plurality of photoelectric convertor elements. For example, when the effective width of the light receiving face is assumed to be 100 xcexcm, spread of the stimulated emission can be suppressed not larger than four pixels, and accordingly deterioration in sharpness of the image due to dispersion of the stimulated emission can be prevented.
When the effective width of the photoelectric convertor element as measured in the direction perpendicular to the longitudinal direction of the line sensor is 20 to 300 xcexcm, the effect of formulae (1) and (2) is enhanced.
Further, when a stimulating light cut filter which is permeable to the stimulated emission and impermeable to the stimulating light is disposed between the line sensor and the stimulable phosphor sheet, the amount of the stimulating light impinging upon the photoelectric convertor elements can be further reduced, whereby the contrast of the image can be further enhanced.
Further, when the condenser lens is larger in the distance of dispersion of incident light on the condenser lens in the sub-scanning direction of the light receiving face of the line sensor than that in the main scanning direction of the light receiving face of the line sensor as measured on the light receiving face of the line sensor, more stimulating light can be caused to go outside the light receiving face while deterioration of stimulated emission collecting efficiency can be suppressed in the sub-scanning direction and at the same time, dispersion of the stimulated emission in the main scanning direction over a plurality of photoelectric convertor elements can be suppressed, whereby deterioration in sharpness of the image can be prevented.